docs: def check_nonparametric_sources(fname, smodel, investigation_time):
      """
      :param fname:
          full path to a source model file
      :param smodel:
          source model object
      :param investigation_time:
          investigation_time to compare with in the case of
          nonparametric sources
      :returns:
          the nonparametric sources in the model
      :raises:
          a ValueError if the investigation_time is different from the expected
      """
      # NonParametricSeismicSources
      np = [src for sg in smodel.src_groups for src in sg
            if hasattr(src, 'data')]
      if np and smodel.investigation_time != investigation_time:
          raise ValueError(
              'The source model %s contains an investigation_time '
              'of %s, while the job.ini has %s' % (
                  fname, smodel.investigation_time, investigation_time))
      return np